This invention relates to generally to analog and mixed signal (analog and digital) integrated circuits, and in particular to bandgap voltage references used in analog and mixed signal integrated circuits.
Reference voltages are required for a variety of purposes. For example, reference voltages are used to bias circuits or to supply a reference to which other voltages are compared. Bandgap voltage references are known in the art, and provide a reference voltage that is quite stable over a range of temperatures. The basic operation of a bandgap voltage reference follows the concept of developing a first voltage with a positive temperature coefficient, combining that voltage with a second voltage having a negative temperature coefficient, and relating the two voltages in a complementary sense such that the resultant composite voltage has a very low temperature coefficient, approximately zero. The voltage produced by bandgap voltage references is related to the bandgap, which for silicon is approximately 1.2 V. Hence, the name for these references.
One known type of bandgap reference is the Brokaw bandgap reference. An example of a Brokaw bandgap reference 10, shown in FIG. 1, includes a pair of bipolar transistors Q2 and Q1 having their base terminals connected together (although in some Brokaw references there may be a resistor connected between the base terminals). Transistors Q2 and Q1 are operated at different current densities, referring to the current flowing through the emitters. In this example, transistor Q1 is operated at a smaller current density. The operation of Q2 and Q1 at different current densities can be achieved in several ways, for example, by transistors Q2 and Q1 having unequal emitter areas but operated at equal currents, by transistors Q2 and Q1 having equal emitter areas and operated at unequal currents, or by some combination of these arrangements. Resistor R1 is connected between the emitters of Q2 and Q1, whose base terminals are connected together (although there could also be a resistor connected between the two bases), and thus a voltage is produced across resistor R1 which is equal to the difference in the base-to-emitter voltages of Q2 and Q1 (xcex94VBE). The current through resistor R1 is therefore proportional to xcex94VBE. Because the current through resistor R1 is proportional to, and perhaps equal to, the emitter current of Q2, the current through resistor R2 is also proportional to xcex94VBE, as will be the voltage appearing across resistor R2.
The base-to-emitter voltage VBE for a transistor has a negative temperature coefficient, governed by the following equation:
VBE=VG0[1xe2x88x92(TT0)]+VBE0(T/T0)+(nkT/q)*ln(T0/T)+(kT/q)*ln(IC/IC0)
Where VG0 is the extrapolated energy bandgap voltage of the semiconductor material at absolute zero (1.205 V for silicon), q is the charge of an electron, n is a constant dependent on the type of transistor (1.5 being a typical example), k is Boltzmann""s constant, T is absolute temperature, IC is collector current, and VBE0 is the VBE at T0 and IC0. The difference in base-to-emitter voltages, on the other hand, has a positive temperature coefficient governed by the following equation:
xcex94VBE=(kT/q)*ln(J1/J2)
where J is current density. Reference voltage VREF generated at the base of transistors Q2 and Q1 thus has a positive-temperature-coefficient component and a negative-temperature-coefficient component. For example, the voltage across resistor R2 (VR2) has a positive temperature coefficient, and the VBE of Q2 has a negative temperature coefficient. Similarly, the voltage across both resistors R2 and R1 (VR2+R1) has a positive temperature coefficient, and the VBE of Q1 has a negative temperature coefficient. An optional voltage divider including resistors RF1 and RF2 is used to achieve an output voltage VOUT which is a reference voltage that is temperature stable but greater than voltage VREF.
Operational amplifier (OA) senses voltages at the collector terminals of Q2 and Q1 and maintains a relatively constant ratio between the currents IC2 and IC1, and thus maintains a relatively constant ratio between the current densities J1 and J2 of transistors Q2 and Q1. Load resistors RL2 and RL1 are connected between a supply voltage VB and the collector of transistor Q2 and the collector of transistor Q1, respectively. For a design having currents IC2 and IC1, equal to one another, load resistors RL2 and RL1 will typically be equal to one another. When the output voltage VOUT drops below a pre-established optimal level, the ratio of collector currents IC2/IC1 is larger than the ratio of resistors RL2/RL1, and thus the input to operational amplifier OA is positive. This causes the amplifier OA output VOUT to increase so that VOUT returns to its optimal level. Conversely, if the output voltage VOUT rises above the optimal level, the feedback action of amplifier OA will have the opposite effect.
In any circuit design, including the prior art Brokaw bandgap reference shown in FIG. 1, electronic noise will be generated during the circuit""s operation. There are various sources of this electronic noise. Two important types of noise generated in bandgap voltage references, and which dictate a minimum quiescent current, are 1/f noise (also known as flicker noise) and wideband noise. In the FIG. 1 circuit, flicker noise is developed at R1 and R2 because of the noise in the base currents of Q2 and Q1 which flow through R1 and R2. The flicker noise level is directly related to the magnitude of these base currents. Wideband noise for VOUT in the FIG. 1 circuit is due to the collector currents of Q2 and Q1. Generally, the higher the collector current, the lower the wideband noise. This illustrates that different circuit designs trade reduction in one type of noise for an increase in another type of noise. Consideration of noise in circuit design is becoming increasingly important, because of the need for lower quiescent currents and also because of ever smaller device feature sizes. Different circuit designs are needed that enable circuit designers to meet more stringent noise requirements.
Generally, the invention is an improved bandgap voltage reference having advantageous noise characteristics. In one aspect, the invention adds two bipolar transistors to a conventional bandgap voltage reference. One of these added transistors is Darlington configured with one of the two bipolar transistors used in a conventional bandgap reference, and the other added transistor is configured similarly with the other bipolar transistor used in a conventional bandgap voltage reference. The configuration is such that a portion of the currents that flow into the collector terminal of the two bipolar transistors of the conventional bandgap reference circuit are diverted away to the respective collector terminals of the added transistors.
In different embodiments, the inventive bandgap reference includes two diode-connected bipolar transistors, or alternatively resistors, coupled between respective emitters of the bipolar transistors used in the conventional bandgap reference and the respective additional bipolar transistors added in accordance with the invention. Different areas of emitters for the bipolar transistor are contemplated, to divert more or less current from the conventionally used bipolar transistors, and to achieve different noise profiles. In addition, the bandgap reference of the present invention may have various design difference known in the art, such as a feedback mechanism, a voltage divider, and a resistor between the base terminals of the bipolar transistors used in conventional bandgap references.
The different embodiments of the invention have one or more of the following advantages. Compared to prior art circuits, the bandgap reference generates lower flicker noise for a given quiescent current used by the reference. The bandgap reference may also generate lower wideband noise. The voltage reference embodiments therefore provide alternative circuit designs with different noise profiles than were previously known, and allow designers to meet more stringent design constraints.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.